Even if you almost never reboot your box it can go into deep sleep when it's not used. It doesn't "heat the planet" significantly when doing so.
Actually it would be interesting to calculate whether frequent reboots don't use more energy in sum than putting the machine into sleep. Does anybody have numbers on that?
In mobile phones similar optimizations where done at least when battery sizes were well below 1000mAh. I think race to sleep was very competitive. So you run at full power for a short time and then you go to a deep sleep mode.
Discaimer: Have not worked with mobile phones for 10+ years.
Typical boot is somewhere.. okay, lets take 30 seconds for all boot shenanigans. Okay, 1 min for ease of count.
That's 0.0694444444444444% a day.
Doubt it.
Even monster servers (like 4CPU and TBs of RAM) boot up in 10 minutes and that's only 0.69% of the day.
Modern systems don't gobble all up all available power at the startup and pretty quickly go to a power controlled mode.
Sure if you have some monster with a dozens of HDDs then there could be some nuances, but at that point you surely don't reboot often anyway
Disclaimer: I don't meet the 8 hour goal and the unattended boot of my office machine setup has been incomplete for 5 years...
Edit: We do also nightly builds, but they take between 5 minutes and less than 3 hours.